Page 116 - Carbonate Facies in Geologic History

Silurian Buildups-Paleotectonic Settings 103

A food chain (trophic) analysis is suggested by Toomey and Finks (1969). The
sponges, stromatoporoids, and bryozoans are regarded as suspension feeders,
filtering organic matter from moderately quiet water. The algae grew among
them. The only sessile benthos resembling modern macrophagous carnivorous
coelenterates would be the tabulate corals. Accessory dwellers include the trilo-
bite Glaphurus, a detritus feeder, the very abundant suspension feeding pelmato-
zoans, and the brachiopods living in channels and perhaps on upper mound
surfaces. Grazers common on the mounds included the large gastropod M aclur-
ites. Numerous cephalopods may have been the only carnivorous predators. The
same gastropod and nautiloid association is seen in the Lower Ordovician algal-
sponge buildups.
Middle Ordovician mounds of the same size range have been described by
Alberstadt and Walker (1973) in the Carters and Holston formations of Tennes-
see. Holston mounds are faunally monotonous. They begin with a stabilization
phase of pelmatozoan debris and small branching bryozoans and change upward
through a dominance of branching to encrusting bryozoans. The Carters organic
buildups are more complex. Sediment with pelmatozoan debris was apparently
stabilized by upright branching Receptaculites forms (dasycladaceans), encrusting
bryozoans, and pelmatozoans. This earlier assemblage was colonized by lami-
nated stromatoporoids which were followed by a diversified assemblage of domi-
cal corals, laminated stromatoporoids and upright branching dasycladaceans,
pelatozoans, and ramose bryozoans. Finally, laminated stromatoporoids domin-
ated the top of the mound as did encrusting bryozoans in the Holston Forma-
tion.
As with all other older Paleozoic mounds on shallow shelves, hydrologic and
depth controls are deemed less important than the progressive creation of sites for
growth concurrent with an increase in diversity of biological communities (Alber-
stadt and Walker, 1973; Alberstadt et al., 1974). The sequence is outlined as
follows:

Table IV-2. Biological evolution in organic buildups
Encrusting and laminate forms No diversity, one form dominant
Diversification phase. Very high diversity
Upright and encrusting forms dominating
Colonization phase Diversity increasing
Stabilization phase Less diversity
Upright forms in tangled masses
Pelmatozoans acting as barnes
for lime mud accumulation

Silurian Buildups-Paleotectonic Settings

Silurian coral-stromatoporoid micritic mounds and ecologic reefs are among the
best-known in the world due principally to careful and extensive study in two
localities: the Gotland Island exposures off the Swedish Baltic coast (A. Hadding,